How do magicians make people float?

Answer 1

They don't. Magic is a performing art that entertains an audience by creating illusions of seemingly impossible or supernatural feats, using purely natural means.

So it is possible that ropes are used to hoist someone up and make the appear to float. Depending on the distance involved and the lighting conditions, the ropes might not be too visible. The magician could use sleight of hand and misdirection to get the audience to ignore any rope, strings, or other conveyances. Also, if nothing else, they could use dry ice to produce "smoke" to help obscure the rigging.

Now if you mean "magicians" as in sorcerers, then they would need supernatural "help" to make things work. While this type of magic has not been proven as existing, it is conceivable that if there is a supernatural world, it might contain beings that are not bound to Earth and may assist someone in doing what would be impossible any other way.